The present invention relates to a semiconductor circuit device having an insulated gate semiconductor device. More particularly, the invention relates to a device having a power MOSFET, an IGBT (insulated gate bipolar transistor) or the like. More particularly, the invention relates to an insulated gate semiconductor device containing a control circuit such as a negative gate voltage protection circuit or an overheat protection circuit formed on the same chip.
Semiconductor circuit devices having large current capacities such as power MOSFETs are required in the fields of circuits, printers, hard disk drive, portable telephones, car electronics or the like. In these fields, semiconductor circuit devices are required which are immune from both overheating and negative gate voltages.
A conventional insulated gate semiconductor device containing a control circuit of this kind can be represented by, for example, a power MOSFET containing a control circuit such as an overheat protection circuit formed on the same chip to improve reliability as disclosed in, for example, Japanese Patent Laid-Open No. 7-58293. According to this reference, a gate resistor is connected between an external gate terminal and an internal gate terminal of the power MOSFET, and, in addition, a protection MOSFET for turning off the gate of the power MOSFET is connected between the internal gate terminal and an external source terminal. The gate resistor is for avoiding a short-circuit between the external gate terminal and the external source terminal when a positive voltage is applied to the external gate voltage. When the chip temperature rises beyond a predetermined temperature, therefore, the protection MOSFET is turned on to cause a gate current flow to the gate resistor to lower a potential of the gate of the power MOSFET, and the power MOSFET is turned off before the power MOSFET is broken.
According to this reference, the control circuit formed on the same chip is constituted by using-an element having a self-isolation structure formed in the drain region of the power MOSFET in order to suppress an increase in the number of the processing steps. Therefore, though the cost can be suppressed, there arises a probability that a leakage current flows from the external drain terminal to the external gate terminal through a parasitic npn bipolar transistor that exists between the drain of the protection MOSFET and the drain of the power MOSFET when the gate voltage becomes negative. In order to cope with this, therefore, a cut-off diode for suppressing the base current of the parasitic npn bipolar transistor has heretofore been connected in series with the protection MOSFET, and, besides, a protect diode for preventing the above cut-off diode from being broken down has been connected between the external gate terminal and the external source terminal.
In order to operate the power MOSFET containing the overheat protection circuit at a high frequency, furthermore, Japanese Patent Laid-Open No. 6-244414 discloses an example of using a cut-off MOSFET instead of the above-mentioned gate resistor. The cut-off MOSFET is also for avoiding a short-circuit between the external gate terminal and the external source terminal when a positive voltage is applied to the external gate voltage. This reference employs the cut-off MOSFET whose body potential is fixed to the source terminal voltage. It is noted that here, and throughout the rest of this description, the term "body" means a region in which the MOSFET being referred to is formed.